1. Field of the Invention
The present invention relates to a coupling structure of a tap and a can in a rechargeable battery and a method of forming the rechargeable battery.
2. Description of Related Art
Since rechargeable batteries have many advantages such as rechargeability, miniaturization, and high capacity, there has been rapid development of rechargeable batteries as they have become more widely used in recent years. Rechargeable batteries can be classified into nickel-hydrogen (Ni-MH) batteries and lithium ion (Li-ion) rechargeable batteries depending on electrode active materials.
A liquid electrolyte, solid polymer electrolyte, or gel type electrolyte can be used as the electrolyte of the lithium rechargeable battery. Also, lithium rechargeable batteries can be classified into can types and pouch types depending on the container into which an electrode assembly is inserted.
In can type lithium rechargeable batteries, the electrode assembly is located in a can formed from metallic materials such as aluminum-containing metal by the use of a deep drawing method and the like. Typically, liquid electrolyte is used in can type rechargeable batteries.
Can type rechargeable batteries can be further classified into polygonal can types and cylindrical can types. The polygonal can type is formed from a container in a hexahedron shape or in a thin shape in which the edges of the hexahedron are rounded. The cylindrical can type is widely used in high-capacity electronic and electrical devices, in which a plurality of rechargeable batteries are combined into a single battery pack.
FIGS. 1 and 2 are a front cross-sectional view and an exploded perspective view, respectively, illustrating a structure of a conventional cylinder type rechargeable battery and a method of forming a cylinder type rechargeable battery will be described with reference to FIGS. 1 and 2. First, two electrodes 25 having a rectangular plate shape and separators 21, 23 interposed between the electrodes 25 to prevent short circuits between the two electrodes 25 are stacked and wound in a jelly roll configuration to provide an electrode assembly 20. Each electrode is formed by coating active material slurry on a charge collector made of a metallic foil.
Uncovered areas absent the slurry are located at either end of the charge collector. The uncovered areas are provided with electrode taps 27, 29 for each electrode plate. The electrode taps 27, 29 are electrically connected to a cylindrical can 10 and a cap assembly 80 insulated from the cylindrical can 10 to form a part of a path for connecting the electrode assembly to external circuits during charging and discharging of the rechargeable battery. From the electrode assembly 20, one electrode tap is drawn upward to an opening of the cylindrical can 10 and the other electrode tap is drawn downward.
The electrode assembly is inserted into the cylindrical can 10 through the opening with upper and lower insulating plates 13a, 13b. Then, beads for preventing floating of the electrode assembly in the can are formed in the cylindrical can 10 and electrolyte is injected into the cylindrical can 10. An insulating gasket 30 is provided on the inner side of the can in the vicinity of the opening and a cap assembly 80 for capping the opening is provided on the inside of the gasket 30.
In the cap assembly 80, a bent assembly, a positive thermal coefficient (PTC) element 60, and a cap-up 70 having an electrode terminal are included. The bent assembly typically includes a bent 40 at the lower side and a current interrupt device (CID) 50 which would be fractured in combination with the bent 40 to cut off a current path.
Subsequently, clamping work is performed to seal the cylindrical can 10 by pressing the sidewall of the opening of the cylindrical can 10 toward the center of the can using the cap-up 70 inserted into the gasket 30 as a stopper. In addition, tubing work is performed to form an exterior of the battery.
In connecting the electrode taps 27, 29, the electrode tap 29 extending downward is welded to a bottom surface of the cylindrical can with the lower insulating plate 13b therebetween and the electrode tap extending upward 27 is welded to the bent 40 through a hole in the upper insulating plate 13a. 
The upward electrode tap 27 has a length sufficient to easily weld the upward electrode tap 27 and a protrusion 42 of the bent 40 to each other. The electrode tap 27 is bent and the bent assembly is inserted into the opening of the cylindrical can in which the gasket 30 is provided. This work requires a space between the bent assembly and the electrode assembly 20.
Since extra length of the upward electrode tap 27 is difficult to dispose after welding, the length of the upward electrode tap is determined in a workable range. The portion of the upward electrode tap 27 remaining after welding may be positioned in a space between the cap assembly and the electrode assembly. The upper insulating plate 13a may serve to prevent a short circuit between the upward electrode tap 27 and the other electrode 25 of the electrode assembly.
A center pin 18 shown in FIG. 2 may be provided in a hollow for winding the electrode assembly 20.
In welding the downward electrode tap 29, in the partial section shown in FIG. 3, the electrode assembly 20 is inserted into the cylindrical can 10. Resistance welding is then performed by inserting a welding rod 115 into a hollow space at the center of the electrode assembly 20 and allowing current to flow in the welding rod 115 such that the electrode tap 29 comes in close contact with the bottom surface of the can 10. A welding point 291 is formed at a position between the electrode tap 29 and the can 10 with which the narrow end of the welding rod 115 comes in contact. Since the welding rod 115 is inserted into a small space at the center of the electrode assembly for the welding, it is difficult to perform the welding while moving the welding rod. Thus, a one-point welding 291 is performed.
A strong weld is typically needed between the electrode taps of the electrode assembly and the safety bent or the can. If the weld is not strong, the electrical connection between the electrode tap and the safety bent or the can is often made through a surface contact. The electrical connection with the surface contact generates contact resistance which is greater than the welding resistance. Accordingly, the internal resistance of the battery increases and the charging and discharging efficiency decreases.
Specifically, in case of the one-point welding on the bottom surface of the can, the strength against tension easily satisfies a predetermined criterion. However, when the electrode assembly rotates inside the can, the strength against the rotation is very weak because there is only one welding point formed at the center of the electrode assembly. Accordingly, the weld may be destroyed, rapidly increasing the internal resistance. Thus, there is a need for a rechargeable battery in which the reliability of the electrical connection between the bottom surface of a can and a downward electrode tap is improved.